Over the past year, the number of cementing and zonal-isolation papers was much lower than usual because of the cancellation and postponement of several conferences. The overall impact, however, was mitigated by the transformation of several conferences to online events. I attended one virtual event and was pleasantly surprised with the value of the technical discussions that immediately followed each paper presentation and the willingness of authors to follow up on discussions after the conference. The development of new technology can take a significant amount of time and resources, and full worldwide introduction may take many years, giving rise to a commercial risk for the technology developer. One paper published last year described the first case history of self-healing cement technology in one geographical area more than 10 years after the publication of the first field application of the technology. In the October 2021 issue of JPT, SPE President Kamel Ben-Naceur discussed the importance of collaboration in solving technical problems quickly and efficiently. Three of the papers selected in this section are from collaborations between multiple companies pooling their expertise and resources to develop solutions for complex problems that could not be addressed efficiently by individual companies. Papers SPE 204015 and SPE 208782 issue from consortia whose goals are to improve the efficiency and reliability of plugging-and-abandonment operations. Paper SPE 204103 is a jointly funded collaboration evaluating CO2-resistant cement with a new experimental methodology to accelerate chemical interactions between the cement and CO2, thus shortening the validation time. The commercial risks associated with the incremental developments of existing technology can be acceptable more readily to individual companies, although this does not mean that efficient internal teamwork is not required. The three other papers selected are all supported and authored by individual companies, but each is a collaboration between multiple people with different expertise—experimental and poromechanical modeling expertise in paper SPE 206144, chemical synthesis and microfluidics expertise in paper SPE 207472, and cement chemistry and experimental expertise in paper SPE 208679. Recommended additional reading at OnePetro: www.onepetro.org. SPE 204103 Evaluation of New Innovative Cement Blend for Enhanced CO2 and H2S Resistance by Gunnar Lende, Halliburton, et al. SPE 206144 Advanced Cement Mechanical Integrity for Thermal Wells by Manh-Huyen Vu, Curistec, et al. SPE 207472 Associative Microgels, New Self-Adaptive Systems To Control Fluid Loss in Well Cementing by Arnaud Cadix, Solvay, et al.

Understanding the effects of high temperature (HT) and high pressure (HP) conditions on the microstructure of cement-based materials is critical to the construction and safe operation of deep oil and gas wells. Under such conditions, the persistence of calcium-silicate-hydrate (C-S-H) gel is compromised by ongoing crystallization that, if not controlled, may adversely affect the durability of the cement sheath. This work investigates the effect of silica content > 35% by-weight-of-cement (BWOC), silica partide size, and solid volume fraction (SVF) on the microstructure and phase composition of cement-silica blends cured hydrothermally at 200 degrees C and 20.7 MPa. The results of X-ray diffraction and electron microprobe analysis revealed significant impact of these three mix design parameters on the final phase assembly, and on the conversion rate of semi-crystalline C-S-H to gyrolite and 11 angstrom tobermorite. Incorporation of more fine siliceous material suppressed dissolution of coarse silica particles, resulting in a matrix with improved homogeneity and dominated by fine gel pores. Mixes with lower SVF showed greater formation of 11 angstrom tobermorite, a higher degree of crystallinity and/or greater crystallite size. Prolonged HTHP curing of all systems (up to three months in this study), irrespective of the initial SW, increased the fraction of capillary pores, indicating void coalesce caused by crystal growth. However, we find that this coarsening is less pronounced in systems with less pore space available for crystallization.

An alkali-activated blend of aluminum cement and class F fly ash is an attractive solution for geothermal wells where cement is exposed to significant thermal shocks and aggressive environments. Set-control additives enable the safe cement placement in a well but may compromise its mechanical properties. This work evaluates the effect of a tartaric-acid set retarder on phase composition, microstructure, and strength development of a sodium-metasilicate-activated calcium aluminate/fly ash class F blend after curing at 85 °C, 200 °C or 300 °C. The hardened materials were characterized with X-ray diffraction, thermogravimetric analysis, X-ray computed tomography, and combined scanning electron microscopy/energy-dispersive X-ray spectroscopy and tested for mechanical strength. With increasing temperature, a higher number of phase transitions in non-retarded specimens was found as a result of fast cement hydration. The differences in the phase compositions were also attributed to tartaric acid interactions with metal ions released by the blend in retarded samples. The retarded samples showed higher total porosity but reduced percentage of large pores (above 500 µm) and greater compressive strength after 300 °C curing. Mechanical properties of the set cements were not compromised by the retarder.

A novel approach for the chemo-mechanical characterization of cement-based materials is presented, which combines the classical grid indentation technique with elemental mapping by scanning electron microscopy-energy dispersive X-ray spectrometry (SEM-EDS). It is illustrated through application to an oil-well cement system with siliceous filler. The characteristic X-rays of major elements (silicon, calcium and aluminum) are measured over the indentation region and mapped back on the indentation points. Measured intensities together with indentation hardness and modulus are considered in a clustering analysis within the framework of Finite Mixture Models with Gaussian component density function. The method is able to successfully isolate the calcium-silica-hydrate gel at the indentation scale from its mixtures with other products of cement hydration and anhydrous phases; thus providing a convenient means to link mechanical response to the calcium-to-silicon ratio quantified independently via X-ray wavelength dispersive spectroscopy. A discussion of uncertainty quantification of the estimated chemo-mechanical properties and phase volume fractions, as well as the effect of chemical observables on phase assessment is also included.

With ever more challenging (T,p) environments for cementing applications in oil and gas wells, there is a need to identify the fundamental mechanisms of fracture resistant oil well cements. We report results from a multi-technique investigation of behavior and properties of API class G cement and silica-enriched cement systems subjected to hydrothermal curing from 30°C to 200°C; including electron probe microanalysis, X-ray diffraction, thermogravimetry analysis, electron microscopy, neutron scattering (SANS), and fracture scratch testing. The results provide a new insight into the link between system chemistry, micro-texture and micro-fracture toughness. We suggest that the strong correlation found between chemically modulated specific surface and fracture resistance can explain the drop in fracture properties of neat oil-well cements at elevated temperatures; the fracture property enhancement in silica-rich cement systems, between 110° and 175°C; and the drop in fracture properties of such systems through prolonged curing over 1year at 200°C.

This work is focused on the chemo-mechanical characterization of cement-based materials. A novel approach is presented which combines the classical grid indentation technique with x-ray energy dispersive microanalysis method. Two oil-well cement systems with different combination of silicious filler are examined, both, from mechanistic and chemical points of view. The characteristic x-rays of major elements, silicon, calcium and aluminum are measured over the indentation region and mapped back on the indentation points. Measured intensities together with indentation hardness and modulus are considered as the set of observables used in the deconvolution analysis. Inference of the number of phases, their average properties and clustering of experimental observations are executed in the framework of Finite Mixture Models with Gaussian component density function. The calcium-silica-hydrate gel is successfully isolated at the indentation scale from other phases of cement hydration and its mechanical response linked to the calcium-to-silicon ratio quantified with x-ray wavelength dispersive spectroscopy.

Abstract With the search for hydrocarbons moving to more extreme environments, includingdeepwater, one of the challenges associated with cementing is ensuring thelong-term integrity and mechanical properties of the cement at hightemperatures (HT). To avoid strength retrogression at temperatures above about 110°C, silicais added to the cement. This makes the hydration process (setting and curing ofthe cement) more complex, as initially formed hydration products are replacedby more stable phases over time. To study the nanostructure and mechanicalproperties of HT-cured cement, we applied a variety of techniques includingsmall-angle neutron scattering (SANS), transmission electron microscopy (TEM), nanoindentation, and micro-scratch testing on small cement samples andinvestigated their structure and properties under a variety of conditions. Weobserved that, at HT, there is a general coarsening of the nanometer-scalestructure of the set cement paste over time, with associated degradation of the properties. We showthat the rate of coarsening depends strongly on the initial curing conditions, providing possible strategies for improving the properties and performance ofHT-cured cement. The findings may have particular application to geothermal wells andsteam injection wells. 1. Introduction Oilwell cementing, in which the annular space between the drilled formationrock and the steel casing pipe is filled with a cement slurry, presentssignificant challenges due to the wide range of environmental conditionsencountered. In particular, the high temperatures and pressures experienced in deep wells generatesignificant changes in the chemical nature and morphology of the hydrationproducts (Taylor, 1997). At near-ambient conditions, the main hydration productof Portland cement is calcium-silicate-hydrate (C-S-H) gel with a Ca/Si molar ratio (C/S)of about 1.7 (Richardson and Groves, 1992, Taylor, 1997). The C-S-H phase hasan amorphous or poorly crystalline nature and a high specific surface area, andis responsible for the generally excellent engineering properties of cement andconcrete cured under normal conditions. As the curing temperature increases toabout 110!C, the C-S-H phase becomes increasingly crystalline but retains itsgeneral structure and properties. Above 110!C, a different phase, alpha-dicalcium silicate hydrate (Ca2SiO2(OH)2) becomes stable and replacesC-S-H. As the C-S-H phase converts to Ca2SiO2(OH)2, the permeability increasesand the strength decreases. To avoid this, oilwell cements for use above 110!C contain a significant amountof silica (approximately 35% by weight of cement) to lower the C/S of thehydration products to about 1. Under these conditions, a range of crystallinecalcium silicate hydrate minerals with generally acceptable engineering properties can form(Taylor, 1964, Kalousek, 1968, Eilers and Root, 1976). While the equilibrium phases over a wide range of conditions are wellestablished (e.g. Shaw et al., 2000, Meller et al., 2009), metastable phasesoften form and linger for extended periods of time. In addition, the morphologyof the products are also highly variable. Thus, it is quite difficult to predict the nature of thehydration products that will develop under particular conditions of time, temperature and pressure. Here we apply novel techniques to investigate theproperties of cement/silica blends cured at high temperature and pressure. Small-angle neutron scattering (SANS) and transmission electron microscopy(TEM) are used to obtain information about the nanometer-scale structure, andnanoindentation and microscratch testing are used to determine the mechanicalproperties.

Recent advances in scratch test analysis provide new ways to relate measured scratch test properties not only to strength properties but fracture properties of materials as well. Herein, we present an application of such tools to oil well cements cured at high temperatures and pressures. We find a concurrent increase of strength and toughness of different oil well cement baseline formulations which we relate to the water-to-binder ratio for a series of cementitious materials prepared with cement and silica flour. The scratch test thus emerges as a self-consistent technique for both cohesive–frictional strength and fracture properties that is highly reproducible, almost non-destructive, and not more sophisticated than classical compression tests, which makes this ‘old’ test highly attractive for performance-based field applications.